Method for electrolytically refining aluminum



Patented Jan. 15, 1952 METHOD-%FOR ELECTROLYTCALLY: REFININ G ALUMINUM- Pierre 'l'cl 'eh Verneuil-sur-Seine, France, as-

sgno' to Pechiney-Compagnie* de* Prodits 4 Cl'miques -et Electrom society r etallurgiques, a French;

Application July 19, 1946, SerialNo.: 684571:

. m France December 3; 1945 The* object of my inventiorr is to -recover in a stated-'great purity'the a'lum iniun contained- 'in the scrapof* certain alloys of said' metal, b'y adapting' to* said operation' the method disclbsed in .the U. S. Patent No. 2,034,3 39 dated March 17; '1936,"for the refining of aluminiumr Accordin -to"this--method; there -are superpose'd in' thetmolten stateinside' an e'lectrolytic vat a layer of heavy *aluminium alloy serving as an ano de' and a 'layer'o'f a' selectve moitenelectrolyte over which the` refined aluminiumformin ;at the cathode fioats.

The electrolyte comprisesamixture of aluminum' fluoride, sodium fiuoride and' an' a'lkalineearth ch'l'oride; the-ratioof the amount' of aluminium" fluoride to that of sodium-= fiucride be'- ing' greater than the 'corresponding ratio' 'ofthe same bodies--in` cryolite', themixture' having "a me1ting"pointbetween '700' C: and 8'00- C? If'the heavy component f' the anodic alloy or* bottom alloy is' Copper; this ali'oy 'should contain" at *least*` 28% -of `copper 'fon' 'its density to `:be sufficient. Its Copper contents should not on' the other hand rise beyond 40% without=detrimentay affecting the purity 'of' the' aluminium. produced at the c'athode During the electrolysi'sy the impuriti'es'ioffi the raw aluminium' accumulate inside-' the::a1loy v at theb'ottomof the'vat': A's'so'on asithe content of Copper in" said alloy reachesi a value near: its up per limit; it`is admixedwwithialuminium inorder to lower- 'the contentst of copp'er; As.. a; "matter- 7 of fact; when i refining aluminium. the=-=leve1- so'f zthe lower alloy mass Varies but littl'e in practice;.for instance between `17= and 2.0i cms; above' the "bot'-` tom of 'a vat" of `3 square-meters"because the.: cone tent's'of iron in said alloy also increa'se and? when they reach too considerableasvaiue;. the=a11oy be comes' pasty. It 'is therefore: necessary 'totremoye thealloy' and* to replace rit bya further load With Iow contents ot -iron This; operationis delicat'e and' costly; the reason th'ereof-is that even with the `greatest care; the"` electrolytic vat' has toc-op &rate drirrg 6 to 10' days befre :it is' possible* to obtain again' at the cathode refined aluminium of a highgrade'of pur-ity; r

Particular difficulties are met with; if'instead 'of -re'fining aluminium, it* de'sir ed to obtain through the` same means refinedf aluminium', starting' from= an ally such as Duraluminc.. for instancecont'aining:

" 'me magnesim contain'ed' n" the' Durammin wou1d'*,a1`so be" a source of trouble because it would 'attack the' electrolysingb'ath; but it`^is5 ais7 easy toeliminate same by previouslymeltin'g the Dura'lumin in* presence of' a salt such as' cryolite;

' for instance; in` the usua'l mann'er* Theailby from which the magnesium ha's been'remov'ed has 'substantially the-same contents of copper as the' original alloy;

If it is desired to extractaluminium -in'astate of high purity' through the above disclosed method i. e; with vohme of' lower'alloy layer varying between' very narrow-hmits; the-contents of'copper' of said alloy reach -rapidly the maxi mum Value allowable before' even' the 'contentsof iron' become troublesom'e: It' is therefore'necessary to often renew the alloy, say about every BOdays. As duringthe 6 or' lo daysfollowing this renewal the' aluminium*produced-doesnt reach the high gra'deof purity'desired; this metal of exceptional quality is` obtaine'd" only during 20 to 24 days out of 30-=whi^ch-renders the=puri fication of scrap Duralumin through this method of Very small economical interest.

The present* inv'ention correct& such* dsadvantages by permitting` elongating considerablythe lapsei of' time during whichthemetal of"1'iighest purity is'obtained;

' According'to' the 'present invention; the a'nodic layer receives, in addition to' the amount' of alloy to betreated corresponding' to the refined aluminium produced at the cathode; an excess of said' 'alloy' such that "while allowinga progressive increase' of the depth' of v thelower a'l loy' layer, the. Iatter'may retain copper contents which remain lower thanthe-maximum vale'consistent with the purty ofxthe rened a1uminiumobtainedi ljform thus: inside-the anodic alloylayer an important' reserve of* Copper which* is removed periodically at long inter-Vals and`-` in large amounts; It is; only- -necess'arythat the a11oy-= to be treateddif'sesrapidly enough in -said' lower layer when it is added.

It"is'`possib1e*t0 allow 'a vari-ati'on in =thedepth of the lower` alloylayer by starting'iroma value lower' than that generallyadopted in the refining of.`a1uminium. It' is*only^necessary-thatthe' ai Ioy to' 'be *treated di'fiuses rapidly enough" in sa-id lowerilayer when it is'addedi Instead`of"a--thick= ness of 17"'to' zo centimeters' as disclosed for the case of the refining of aluminium in a vat'of-' s sq meters* it i'S possibl to admi-t a minimum thickness of -'10` to 12 *centimetrsfbr the lowe' alloy Asto the maximum thickness of said lowe' layer of alloy it is in prineiple only bythe cdnditions of overfiowing of the electrolysis vat. It is however preferable to limit the increase of the thickness of the lower alloy layer to a value which is at the utmost equal to the thickness of the electrolyte layer. In practicein the case of Duralumin; there is addeddaily to the anodic layer an excess of Duralumin of the magnitude of 2 to 4% with reference to the amount corresponding to the production of refined aluminium, the Duralumin bein preliminarily freed from its mag-' nesium by fusion in the presence of cryolite as indicated above.

Appended drawing shows by way of example a cross-sectional View passing centrally through an electrolysis vat adapted for use in the execution of the present invention.

` In the drawings, I designates the body of the' cell, 2 designates the current feeding means for the anodic blocks 3, said means passing through the bottom of the cell, 4 -designates the lower alloy layer Contacting ,with the blocks 3 while 5 designates theelectrolyte layer over which floats the layer 6 of refined aluminium. Cathodic blocks are adapted to dip the layer B carried by rods 8 adapted to be moved vertically through actuation of the hand whee1 sa. The latter is carried by a, shaft 9 mounted on a gantry !0 mounted over the cell, said shaft 9 controlling the rods 8 through any suitable means.

i The left hand side of the drawing shows the cell at the beginning of an Operating period. The thickness h of the heavy alloy layer is then at its minimum value. Towards the end of the operating period as illustrated in the right hand part of the drawing, the respective thickness' of the electrolytelayer and of the layer of refined aluminium have remained unaltered while the anodic alloy layer at the bottom of the vat has increased by a value substantially equal to the thickness of the electrolyte layer, so that its new thickness h' is more than twice h.

Examples i 1. An electrolytic vat .of 3 sq. meters bottom is used for the refinng of aluminum; it is a vat introduced lower alloy layer, at the start of the operation, is 10 cms. thick; said layer is allowed to grow to a thickness of 25 cms. It is possible under such conditions to renew the alloy only every 38l ,days so that it is possible to produce refined aluminium during 370 'to 375 days out of 381. This provides a range of from 30 to 380 days for the renewal of the anodic layer.

The anodic alloy reaches its critical maximum contents of copper before it is saturated with iron. When the contents of copper of said alloy reach their upper limit, it is therefore possible to remove only a part of the alloy and to reduce the contents of copper of the remaining alloy down to thedesired minimum value i. e. 28%

through an .addtion of Duralumin.

of known type in which the thickness of the lower alloy layer, is usually kept between 17 and 20 cms., while aluminum is electrolytically refined in it by known processes. A refining operation of aluminum is carried out with- -at the start-a lower layer of alloy to be refined having the thickness of only 12 cms.; in the course of refining this layer is allowed to reach 'the thickness of 25 cms. i. e. to become about 2 times as thick as it was at the start of the operation( Then instead of being compelled to renew the anodic alloy after 30 days as would have been necessary if usual principles (thickness kept between 17 and 20 cms.) had been respected, it is possible to renew said anodic alloy layer only after 115 days. It is therefore possible to produce refined aluminum during 105 to 109 days out of 115. r

In another operation the thickness of the lower alloy layer was of 10 cms. at the start and was allowed to reach, in the course of the refining, 30 cms., that means the anodic layer became 3 times as thick as it was, suflicient to renew the alloy every 1 80 days and aluminum wasrefined to a highgrade of purity during 170 to 174 days out` of 180. l i

The layer forming the anode is thus permitted to become 2 to 3 times as thick as it was at the refining operation. r

. 2; In an electrolytic vat of ssquare meters, the

The operation continues thus without it being necessary to incorporate copper as such into the bath as the copper introduced by the Duralumin is sufiicient for the requirements.

The iron contents of the lower alloy increase progressively but-have a tendency to reach a limit of 6% 'of which 5% are provided by the Duralumin and 1% by the Operations inside the vat.

As a result 1000 kgs. of Duralumin from which the magnesium has been eliminated and containing: 945 kgs. of Al-40 kgs. of Cu-6 kgs. of Fe- 5 kgs. of Sikgs. of Mn produce 900 kgs. o'f refined aluminium and 100 kgs. of an alloy containing: Al 43%-Cu 40%-Fe 8%-Si 5%-Mn 6%-which latter alloy is eliminated. V

The present invention is by no means limited in its application to Duralumin and it may be used with any aluminium alloys, wherein the proportions of alloyed metals areof comparable magnitude, which means containing more than aluminium, provided their contents of iron do not lead to a more frequent replacement of the lower alloy. In particular it is perfectly applicable to alloys 'containing Zinc, which latter metal may in fact replace wholly or partly copper in the anodic alloy.

WhatIclaim is: i s

1. In a method for electrolytically refining aluminium, which comprisessuperposing in an electrolytic vat, a layer of molten aluminium alloy containing at least one heavy metal and free of magnesium forming the anode, a layer of melted electrolyte mixture of aluminium fluoride, sodium fluoride and an alkaline-earth chloride, the ratio of the amount of aluminium fluoride to that of sodium fluoride being greater than the corresponding ratio of the same bodies in cryolite, said electrolyte mixture having a, melting point be-'- tween 700 C. and 800 C., and a layer of aluminium forming the cathode, additions of aluminium to be refined to said layer forming the anode being made in the course of the refining operation, the steps consisting in daily adding to said molten alloy forming the anode an amount of alloy free of magnesium containing more than 90% of aluminium, greater than the amount of the same which includes the' quantity of daily produced refined aluminium, and removing said layer forming the anode 'for' renewing it only every 30 to 380 days. si r r 2. In a, method for electrolytically refining aluminium which comprises superposing in an electrolytic vat, a layer ,ofmolten aluminium alloy containing at least one heavy metal and' free of magnesium forming the anode, a layer of melted electrolyte mixture of aluminium fluoride, sodium fluoride and an alkaline-earth chloride, the ratio of h mount o? flamir i gi & i hat sodium fluoride being greater than the corresponding ratio of the same bodies in cryolite, said electrolyte mixture havin a melting point between 700 C. and 800 C., and a layer of aluminium forming the cathode, additions of luminium to be refined to said layer forming the anode being made in the course of the refining operation, the steps consisting in daily adding to said molten alloy forming the anode an amount of alloy free of magnesium containing more than 90% of aluminium, greater by 2 to 4% than the amount of the same which includes the quantity of daily produced refined aluminium, and removing said layer forming the anode for renewing it only every 30 to 380 days.

3. In a method for electrolytically refining aluminium, which comprises superposing in an electrolytic vat, a layer of molten aluminium alloy containing at least one heavy metal and free of magnesium forming the anode, a layer of melted electrolyte mixture of aluminium fiuoride, sodium fluoride and an alkaline-earth chloride, the ratio of the amount of aluminium fiuoride to that of sodium fluoride being greater than the corresponding ratio of the same bodies in cryolite, said electrolyte mixture having a. melting point between 700 C. and 800 C., and a layer of aluminium forming the cathode, additions of aluminium to be refined to said layer forming the anode being made in the course of the refining Operations, the steps consisting in daily adding to said molten alloy forming the anode an amount of alloy free of magnesium containing more than 90% of aluminium, greater than the amount of the same which includes the quantity of daily produced refined aluminium, and removing said layer forming the anode for renewing it only every 30 to 380 days, when said layer forming the anode becomes 2 to 3 times as thick as it is at the start of the refining operation.

4. In a method for electrolytically refining aluminium which comprises superposing in an electrolytic vat, a layer of molten aluminium alloy containing at least one heavy metal and free of magnesium forming the anode, a layer of melted electrolyte mixture of aluminium fluoride, sodium fluoride and an alkaline-earth chloride, the ratio of the amount of aluminium fluoride to that of sodium fluoride being greater than the corresponding ratio of the same bodies in cryolite, said electrolyte miXture having a melting point between 700 C. and 800 C., and a layer of aluminium forming the cathode, additions of aluminium to be refined to said layer forming the anode being made in the course of the refining operation, the steps consisting in daily adding to said molten alloy forming the anode an amount of alloy free of magnesium containing more than of aluminium, greater by 2 to 4% than the amount of the same which includes the quantity of daily produced refined aluminium, and removing said layer forming the anode for renewing it only every 30 to 380 days, when said layer forming the anode becomes 2 to 3 times as thick as it is at the start of the refining operation.

5. The method claimed in claim 1 where the thickness of the said layer forming the anode is of 10 to 12 centimeters at the start of the refining operation and when it reaches 25 to 30 centimeters said layer is removedand renewed.

6. The method claimed in claim 2 where the thickness of said layer forming the anode is of 10 to 12 centimeters at the start of the refinng operation and when it reaches 25 to 30 centimeters said layer is removed and renewed.

PIERRE TORCHET.

REFERENCES CITED The following references are of record in the ;file of this patent:

UNITED STATES PATENTS Number Name Date 1,534,317 Hoopes Apr. 21, 1925 1,534,318 Hoopes Apr. 24, 1925 1,782,616 Hulin Nov. 25, 1930 2,034,339 Gadeau Mar. 19, 1936 

1. IN A METHOD FOR ELECTROLYTICALLY REFINING ALUMINUM, WHICH COMPRISES SUPERPOSING IN AN ELECTROLYTIC VAT, A LAYER OF MOLTEN ALUMINUM ALLOY CONTAINING AT LEAST ONE HEAVY METAL AND FREE OF MAGNESIUM FORMING THE ANODE, A LAYER OF MELTED ELECTROLYTE MIXTURE OF ALUMINUM FLUORIDE, SODIUM FLUORIDE AND AN ALKALINE-EARTH CHLORIDE, THE RATIO OF THE AMOUNT OF ALUMINUM FLUORIDE TO THAT OF SODIUM FLUORIDE BEING GREATER THAN THE CORRESPONDING RATIO OF THE SAME BODIES IN CRYOLITE, SAID ELECTROLYTE MIXTURE HAVING A MELTING POINT BETWEEN 700* C. AND 800* C., AND A LAYER OF ALUMINIUM FORMING THE CATHODE, ADDITIONS OF ALUMINUM TO BE REFINED TO SAID LAYER FORMING THE ANODE BEING MADE IN THE COURSE OF THE REFINING OPERATION, THE STEPS CONSISTING IN DAILY ADDING TO SAID MOLTEN ALLOY FORMING THE ANODE AN AMOUNT OF ALLOY FREE OF MAGNESIUM CONTAINING MORE THAN 90% OF ALUMINUM, GREATER THAN THE AMOUNT OF THE SAME WHICH INCLUDES THE QUANTITY OF DAILY PRODUCED REFINED ALUMINUM, AND REMOVING SAID LAYER FORMING THE ANODE FOR RENEWING IT ONLY EVERY 30 TO 380 DAYS. 